Various object detection devices are known in the prior art, and one such device is shown in U.S. Pat. No. 4,293,852 (Rogers). This shows a detection device where there are metallic electrode strips 10 and 11 placed side by side and the article 15 which is to be protected from theft or other removal overlies the electrodes 10 and 11. The article 15 is metallic, or is arranged to incorporate metal at or near its base, for example, by a sticker or tag 5 which is attached to the article and is made of metal, or incorporates metal. There is a sensing circuit responsive to the capacitance present between the two conductors, this capacitance being modified by moving the metallic object (or the metallic tag attached to the object) away from the two strips 10 and 11.
U.S Pat. No. 4,836,033 (Seitz) shows a capacitive force measuring cell built up in sections to measure various forces. The capacitor surfaces are movable relative to each other against elastic resetting forces of the dielectric.
U.S. Pat. No. 4,386,386 (Akita) shows a capacitive sense loader that is in the form of coil spring elements. The change of capacitance between several coils, or between a coil and fixed element, is directed to a sensing device.
U.S. Pat. No. 4,273,204 (Gillan) shows a capacitive load cell that is used in a weighing scale or the like.
U.S. Pat. No. 3,445,835 (Fudaley) shows an article sensor in which an oscillator outputs to an antenna. If there is a change in the electronic environment near the antenna, a detectable frequency shift takes place and the detector is activated.
U.S. Pat. No. 3,230,519 (Metz) shows a capacitive article detector that makes use of a capacitance bridge to detect changes in the article presence.
U.S. Pat. No. 2,062,269 (Knowles) provides a detector in which a cold cathode tube grid is connected to a nongrounded metal article. If the article is moved, grounded, or a large body passes near, the voltage at the grid changes and the alarm is activated.
U.S. Pat. No. 2,064,980 (Kelly) shows a detecting system which detects changes in capacitance. There is a means for providing a constant frequency electrical supply, and a circuit including an antenna connected to and energized by the electrical supply. The rate of change of voltage across a portion of the circuit is detected.
U.S. Pat. No. 3,824,460 (Gustafson) shows a capacitive sensing system adapted for use in detecting the presence of a liquid on a floor in which a sensing probe comprises a pair of encased wires held essentially parallel to each other by a plurality of spaced webs which are an extension of casing of the wire. The probe is held flat on the floor over a considerable length so that leakage anywhere along the probe will result in capacitance change which may be sensed, and an appropriate alarm activated in response thereto.